1.Field of the Invention
The present invention relates to single stage electronic energy converter operated from an alternating power line, and capable of supplying, at the output, a load such as gas discharge lamp.
2. Description of Prior Art
The electronic energy converters, or as sometimes called "switching power supplies" need to operate directly from the alternating power line. Electric utility companies are setting requirements for specific groups of electricity-powered appliances in regards to power quality drawn by these appliances.
The electronic ballast, as one of the appliances, is used in large quantities in lighting fixtures. In general, to meet the industry requirements in regards to power quality, an electronic ballast has to meet two fundamental requirements: (i) draw power from the power line with a power factor (PF) of at least 0.9, (ii) draw current from the power line with a total harmonic distortion (THD) of less than 20 percent.
The electronic ballast has to meet other requirements related to compatability with a lamp-load. The electronic ballast shall provide lamp current crest factor of less than 1.7, where the "crest factor" is equal to a peak magnitude of the lamp current divided by its effective (RMS) value. This can be, in many situations, related to maximum allowable modulation of the lamp current magnitude, which is responsible for light flicker. It is desirable to have a constant power delivered to the lamp-load over the entire cycle of the voltage supplied by the power line.
In order to convert the low frequency alternating voltage of a conventional power line (120 Volts/60 Hz or 220 Volts/50 Hz) to a high frequency (typically from 10 to 100 kHz) alternating voltage or current source, one has to rectify the signal from the power line, to a DC voltage which later is converted, by switching transistors, to the high frequency source.
Conventional off-line rectifiers have a capacitive smoothing filter located beyond a diode rectifier circuit. This smoothing capacitor causes harmonic distortion of the current waveform during periods in which the rectified output is higher than the voltage over the smoothing capacitor, and during which time the capacitor charges up. This charging time, or conduction angle, is very small if large capacitor is used, and all the required charge has to be loaded into the capacitor in a short period of time. This results in a large current output from the rectified supply during the short conduction angle, and causes the current spikes in the rectified supply. These current spikes increase the harmonic content of the power supply, and when a number of ballasts are being used, this increased harmonic distortion causes a poor power factor in the supply. This situation is not accepted upon by electricity supply authorities, and it causes interference with other electrical equipment.
Techniques for improving power factor include passive waveform shaping methods. One of them is described in U.S. Pat. No. 5,150,013 issued to Bobel. This method requires an inductor to operate in a resonant mode with a capacitor, and the resonant frequency is approximately 180 Hz when power line frequency is 60 Hz. This method is very inexpensive and reliable. However, the inductor must be large in size.
It is also known to use a storage conversion principle, whereby an inductor is controlled at high frequency in order to allow charging of the smoothing capacitor over a wide conduction angle. The system however requires a control circuit for the storage converter, known also as "boost converter", in order to regulate the discharge of energy from the storage inductor. Such a use of the storage conversion principle requires additional noise filtering because of large amount of noise is being generated by switching devices. The circuit is very complex and expensive to produce. Furthermore, the second stage converter is necessary to convert the DC voltage source to the high frequency alternating voltage or current source. This type of circuit is described in U.S. Pat. No. 5,049,790 issued to Herfurth. It is also known to use a single stage converter which draws near sinusoidal current from the sinusoidal power line source, and delivers high frequency current to the lamp-load. In this principle, which uses resonant oscillatory circuit having ability to store and release energy, portion of the resonant energy is re-directed from the output to the input of the converter. This method creates large circulating currents within the oscillatory circuits, thus causes large amount of power being dissipated within the converter. The following patents describe single stage inverters which have portion of the energy from the output redirected to the input of the converter, and exibit large amount of power dissapation.:
______________________________________ U.S. Pat. No. Patentee ______________________________________ 4,017,785 Perper 4,109,307 Knoll 4,642,745 Steigerwald et al. 4,782,268 Fahnrich et al. 4,808,887 Fahnrich et al. 4,985,664 Nilssen 4,954,754 Nilssen 5,010,277 Courier de Mere 5,134,556 Courier de Mere 5,113,337 Steigerwald 5,099,407 Thorne 5,103,139 Nilssen ______________________________________
It is highly desirable to have a simple and low cost single stage electronic ballast to solve problems of the above inventions and meet all the industry requirements.
However, this applicant is not aware of any prior art relevant to an integrated, single stage electronic energy converter wherein, the energy used to correct the power factor is not re-directed from the output to the input of the device.